Risers are commonly used to link hydrocarbon wells on the seabed to floating installations or vessels such as oil rigs or ships. A riser is made up of lengths of tubing and is extremely heavy. The surface vessel therefore needs to apply tension to the riser to prevent it collapsing under its own weight. However, in certain sea conditions, for example, as the vessel moves, the applied tension will fluctuate. As the riser is fixed at its lower end to the wellhead assembly on the seabed and at its upper end, by the tensioners, to a floating installation or vessel, it is necessary for motion of the installation caused by wind, wave and tidal action to be accommodated. Consequently, motion compensating means must be incorporated into the tensioning system to maintain the top of the riser within the moon pool of a ship and at rig floor level. This may include a telescopic marine joint or a drill string compensator to compensate for heaving motion while maintaining a predetermined tension to the riser and a flex joint within the riser to compensate for lateral motion of the vessel. The telescopic marine joints used are well known and are referred to herein as slip joints. A typical slip joint comprises concentric cylinders which are arranged to telescope relative to each other, with a dynamic seal provided between them.
However, should the motion compensating means lock up, the tension in the riser will fluctuate. At excessive tensions, it is known for risers to break. This can cause an environmental problem as the riser may be full of hydrocarbons at the time of separation, which hydrocarbons could subsequently leak from the riser.
To counter this problem, risers may be provided with a weak link which has a lower tensile rating than the other components of the riser and, in the event of over tensioning the riser, the riser will separate at the weak link.
WO 03/069112 discloses a sleeve, which is positioned around a riser. The sleeve can move up and down the riser between two raised portions. The sleeve can be attached to blow out preventer rams such that the riser can rotate or move in an axial direction with respect to the blow out preventer. The sleeve is initially fixed to the riser with shear pin, which may fail after attaching the sleeve to a blow out preventer ram.
U.S. Pat. No. 4,424,988 discloses a weak link formed by a weakened portion of a bolt connecting two riser portions. The document discloses two ways of breaking the bolt: first, by a tensile force between the upper and lower riser section; second, by applying a high hydraulic pressure to a chamber within a connector between the two riser portions. The hydraulic pressure within the chamber causes a pressure differential between an annular member and an annular flange, which causes the bolt to fail when a threshold pressure is exceeded. The two riser portions can move with respect to each other after the bolt has failed.
WO 2009/153567 discloses a weak link. Within the weak link, the effects of a variable well pressure are balanced by the application of hydraulic pressure. A pressure application device is provided to apply a coupling force to the weak link in order to counter a separation force applied by well pressure. Separation of the weak link due to well pressure can thereby be avoided.
A problem with the above weak link solutions is that they only provide protection against tensile forces between the upper and lower riser section. Should the motion compensating means lock up during an initial downward motion of a floating installation or vessel, the known solutions offer no protection against compression forces to the riser.
The object of the invention is therefore to provide an improved solution that solves the above problems relating to motion caused by compressive as well as tensile forces and is more reliable in terms of functionality. These objects and others will become apparent from the following description.